Update on Cannabidiol Clinical Toxicity and Adverse Effects: A Systematic Review

Background Compelling evidence from preclinical and clinical studies supports the therapeutic role of cannabidiol (CBD) in several medical disorders. We reviewed the scientific evidence on CBD-related toxicity and adverse events (AEs) in 2019, at the beginning of the spike in clinical studies involving CBD. However, CBD safety remained uncertain. Objective With the benefit of hindsight, we aimed to provide an update on CBD-related toxicity and AEs in humans. Methods A systematic literature search was conducted following PRISMA guidelines. PubMed, Cochrane, and Embase were accessed in October 2022 to identify clinical studies mentioning CBD-related toxicity/AEs from February 2019 to September 2022. Study design, population characteristics, CBD doses, treatment duration, co-medications, and AEs were compiled. Results A total of 51 reports were included. Most studies investigated CBD efficacy and safety in neurological conditions, such as treatment-resistant epilepsies, although a growing number of studies are focusing on specific psychopathological conditions, such as substance use disorders, chronic psychosis, and anxiety. Most studies report mild or moderate severity of AEs. The most common AEs are diarrhea, somnolence, sedation, and upper respiratory disturbances. Few serious AEs have been reported, especially when CBD is co-administered with other classes of drugs, such as clobazam and valproate. Conclusion Clinical data suggest that CBD is well tolerated and associated with few serious AEs at therapeutic doses both in children and adults. However, interactions with other medications should be monitored carefully. Additional data are needed to investigate CBD's long-term efficacy and safety, and CBD use in medical conditions other than epilepsy syndromes.


INTRODUCTION
Cannabidiol (CBD) is a prevalent bioactive compound in the Cannabis sativa plant, second to Δ 9 -tetrahydrocannabinol (THC) but without eliciting psychoactive effects [1][2][3][4].CBD's precise molecular mechanism is still not fully elucidated.Compared to THC, CBD shows a very low affinity for endogenous cannabinoid receptors CB 1 and CB 2 [5] and actually seems to antagonize the CB 1 receptor by allosteric modulation [6][7][8].CBD also may act as an inverse agonist at CB 2 , partially explaining its anti-inflammatory properties [9].Its effects have also been related to the inhibition of fatty *Address correspondence to this author at the Unit of Forensic Toxicology, Section of Legal Medicine, Department of Biomedical Sciences and Public Health, Marche Polytechnic University, Ancona, Italy; Tel: +3932244667; E-mail: fra.busardo@libero.itacid amide hydrolase, the inhibition of adenosine reuptake, the modulation of transient receptor potential channels and the orphan G-protein-coupled receptor GPR55, the facilitation of serotonin receptor 5-HT 1A and glutamatergic mediated neurotransmission, and the inhibition of inflammatory cytokines through receptors such as the peroxisome proliferator-activated receptor-gamma (PPAR-γ) [10][11][12].
Considering the recent approval by the FDA and EMA for childhood epilepsy syndrome and the wide range of pharmacological effects with a broad spectrum of potential clinical use, CBD safety needs to be investigated.In 2019, we reviewed the scientific evidence on CBD-related toxicity and adverse events (AEs) [28].However, most clinical studies involving CBD were published in recent years, providing essential data on CBD safety at specific controlled doses in humans.The aim of the present review was to provide an update on CBD toxicity and AEs, specifically focusing on clinical trials.

MATERIALS AND METHODS
A systematic literature search was conducted according to the PRISMA guidelines [29].Report identification, screening, and assessment for eligibility and inclusion were conducted by two authors independently after agreeing on the search term list.The results were cross-checked after each step of the investigation.

Report Identification
PubMed (US National Library of Medicine), Cochrane, and Embase bibliographic databases were accessed in October 2022 to identify scientific reports of CBD-related toxicity and AEs.Other potential reports were searched from international agencies or institutional websites including Clini-calTrials.gov and US FDA.The search terms were: "cannabidiol", "CBD", or "Epidiolex"; in combination with "adverse effects", "side effects", "adverse reactions", "adverse events", "toxicology", "toxicity", or "safety".Further manuscripts were retrieved through the reference lists of selected articles.Duplicates were excluded based on the articles' DOI using Microsoft Excel (v.2211); the results were manually checked before removal.Only the studies published after February 2019 were included to avoid redundancy with our previously published article [28].

Report Screening and Assessment for Eligibility
From the list of identified reports, clinical studies with a title and abstract mentioning CBD use were eligible for fulltext reading.If a title/abstract was insufficient to understand whether the corresponding report should be included, the report was eligible for full-text reading.Clinical studies on CBD with THC co-administration only and case reports were excluded, as observed adverse events and toxicity can hardly be linked to CBD use.Literature reviews, commentaries, letters to the editor, viewpoints, or erratum; purely analytical (method development and chemical analysis), in vitro or preclinical studies; and reports that were not written in English, Italian, or French were excluded.

Report Inclusion
The full text of manuscripts reporting CBD toxicity or AEs in clinical studies was included.Clinical studies on CBD with THC co-administration; case reports; literature reviews, commentaries, letters to the editor, viewpoints, or erratum; purely analytical, in vitro, or preclinical studies; and reports that were not written in English, Italian, or French were excluded, if not excluded during the report identification and screening steps.
Authorship, date of publication, study title, study design, population characteristics, CBD doses, treatment duration, co-medications, and AEs were compiled.
The studies were compared based on their location and date of occurrence to avoid duplicates.If it arose, data were merged into a single entry in the database.The authors' affiliations, funding sources, and disclosures were verified for each report to avoid potential bias linked to conflicts of interest.

RESULTS
The results of the systematic review are presented in a flow diagram of the literature search (Fig. 1).The initial search strategy yielded a total of 1053 articles that were screened by title and abstract for eligibility.One hundred duplicates were removed.Of the 953 remaining records, 707 were removed as they included preclinical studies, inappropriate study design/population, and THC co-administration.A total of 246 studies were considered for full-text review.Forty-nine reports referring to clinical trials in a recruiting, not yet recruiting, or completed status with no results posted, and 29 abstracts were excluded because of missing results or not enough data on population characteristics and CBD toxicity.Likewise, six proof-of-concept papers and five case reports were excluded as they did not meet the inclusion criteria.Finally, a total of 51 reports met the inclusion criteria of this systematic review (Table 1).Study design, population characteristics, CBD doses, treatment duration, co-medications, and AEs are depicted in Table 1.The database compiling all the retrieved data is available on demand.
In the OLE studies, LGS, DS, and TSC patients who completed the RCTs were enrolled in a long-term open-label extension trial for each pivotal study allowing the evaluation of treatment-emergent AEs related to CBD administration.Across the trials, a pharmaceutical formulation of highly purified CBD (Epidiolex; 100 mg/mL) in oral solution was administered as an add-on treatment to the pre-existing antiepileptic regimen at up to 50 mg/kg/day with a follow-up time of 48 weeks.Most patients reported at least one AE, whose severity was mostly mild or moderate.The most commonly reported AEs in patients with LGS, DS, and TSC were pyrexia and gastroenteric tract disorders, including diarrhea, vomiting, and decreased appetite [40,41,43,47,52].Somnolence, sedation, drowsiness, and convulsion were the most frequent neurological AEs.Overall, the reported AEs, frequencies, and severity, are comparable with previous observations in the RCTs [46,51].Treatment discontinuation was associated with increased liver transaminase and seizures.Elevation of liver transaminase was more frequent in patients taking concomitant valproate and higher dosages of CBD.
An open-label, multicenter, expanded access trial reported similar findings in children with TRE, ineligible to be included in RCTs that only included patients with LGS or DS [42].All 47 enrolled patients experienced at least one AE during the 36-month treatment.Most AEs were mild or moderate in severity.Serious AEs were reported by twelve children; none of them were related to CBD according to the authors.The most common AEs were upper respiratory infections, gastrointestinal disorders (diarrhea, nausea, vomiting), pyrexia, and somnolence [42].A well-tolerated safety profile of oral CBD and a good response rate for controlling seizures was confirmed by an expanded access program trial involving a large cohort of 607 patients (58 with DS, 94 with LGS, and 455 with other TREs) over a 96-week treatment [31].Patients received oral CBD titrated until tolerability to a maximum dose of 25-50 mg/kg/day and were taking a median of three concomitant antiepileptic drugs (AEDs) (range 0-10).After 144 weeks, the safety analysis revealed that 91% of patients reported at least one AE.Most AEs were mild or moderate in severity; diarrhea and somnolence were the most common (30% and 24%, respectively).
A good antiseizure response to CBD, defined as ≥ 50% reduction in seizure frequency, and an acceptable safety profile have been demonstrated by studies evaluating both children and adults with other forms of TREs [32,45], such as developmental and epileptic encephalopathy [37,38,53], refractory childhood-onset epileptic spasm [34], X-fragile syndrome [54], and SYNGAP1 developmental and epileptic encephalopathy [55].Most patients were taking a highly purified oral CBD formulation; in only one study, a transdermal CBD gel at a dosage range of 150-500 mg/day was administered [37].Most AEs were mild or moderate.The most common AEs were related to gastrointestinal (diarrhea, nausea, decreased appetite), and respiratory or neurological disturbances (somnolence and sedation) [37].Overall, a few serious AEs, including increased transaminase level, sedation, and convulsion, were associated with the discontinuation of treatment.
Considering the increasing interest in CBD therapy for epilepsies, concerns regarding potential cognitive effects have been raised by treating physicians.Three open-label trials assessed whether CBD has long-term cognitive effects in both children and adults with TRE [30,33,36].27 adults and 38 subjects between the age of 3 and 19 years with TRE enrolled in open-label CBD studies were evaluated prior to initiating CBD treatment and after one year.There were no statistically significant changes in cognitive functions, as measured by the National Institutes of Health Toolbox Cognition Battery, or functional adaptive skills, as measured by the Adaptive Behavior Assessment System -Second edition after a one-year trial with CBD [33,36].Similarly, no significant CBD-related behavioral effects were found in 39 patients with TRE, including both children and adults, evaluated at baseline and after three months of CBD treatment [30].
A retrospective chart review study investigated a DDI between CBD and mTOR inhibitors, such as everolimus and sirolimus, in a cohort of 25 patients with TSC (18 with everolimus, 7 with sirolimus) [44].The median age at treatment initiation was 17 years (range 3-45 years; 13 children, 12 adults).CBD was orally administered, and follow-up mTOR inhibitor levels were drawn after a therapeutic dose of 5-20 mg/kg/day was achieved.Both everolimus and sirolimus plasma levels were significantly higher in 76% of patients after CBD treatment.AEs occurred in 10 of 25 patients (40%) with diarrhea being the most frequent AEs in three patients (12%).Other AEs included drowsiness, increased and severe mouth sores, increased acne, ankle swelling, sinusitis, abdominal pain, mild elevation of transaminases, and increased phenytoin level [44].
A phase II, two-arm, parallel-group, double-blind, randomized, placebo-controlled trial was conducted to evaluate pharmacokinetic DDI between CBD and stiripentol or valproate in 35 patients with epilepsy aged 16-55 [50].Patients receiving a stable dose of stiripentol or valproate were randomized 4:1 to receive concomitant double-blind CBD or placebo.Oral CBD was administered at a dose of 20 mg/kg/day from day 12 to 26, following a 10-day dose escalation.The combination of CBD and stiripentol led to a small increase in exposure to stiripentol, while the combination with valproate did not result in significant changes in the pharmacokinetics of valproate or its metabolite, 2-propyl-4pentenoic acid (4-ene-VPA).The most common AE was diarrhea; most AEs were mild.Two patients discontinued CBD because of serious AEs (rash (n = 1) in the stiripentol arm; hypertransaminasemia (n = 1) in the valproate arm) [50].
A similar trial investigated potential DDIs between CBD and clobazam in 20 patients with refractory epilepsy aged 18-65 on stable clobazam doses [49].No statistically significant pharmacokinetic changes were found in patients taking concomitant CBD and clobazam for 32 days.A welltolerated safety profile for CBD was reported, with all AEs being mild (31.3%) or moderate (43.8%) in severity.Most common AEs were mild (5 (31.3%)) or moderate (7 (43.8%)).The most common AEs were diarrhea, nausea, vomiting, sedation, and somnolence.One severe AE (6.3%) occurred as a seizure cluster leading to withdrawal.There were two cases (12.5%) of increased transaminase level with concomitant valproate, and three cases (18.9%) required a reduction of CBD dose due to the occurrence of rash, diarrhea, and multiple events of sedation, slurred speech, and word-finding difficulties [49].
CBD is a modulator of the endogenous cannabinoid system, involved in the homeostatic regulation of many physiological processes, such as mood, anxiety, sleep, and neuroprotection, which seem to be involved in mechanisms underlying the pathophysiology of PD.A phase II/III, double-blind, placebo-controlled clinical trial was conducted in 33 PD patients with rapid eye movement behavior disorder.Patients were randomized 1:1 to CBD in doses of 75 to 300 mg or matched capsules placebo and were followed up for 14 weeks [24].CBD showed no reduction of rapid eye movement behavior disorder manifestation in PD patients; AEs were similarly reported in both groups.
A transdermal CBD gel was also evaluated for efficacy and safety profile in a RCTs, double-blind, placebocontrolled, multicenter trial [48].A total of 188 patients (mean age 39.2 (12.78)) with focal resistant epilepsy were randomized (1:1:1) to 195-mg or 390-mg transdermal CBD or placebo daily for 12 weeks, after which they could enroll in an OLE study for up to 2 years.AEs occurred in 50.4% of the CDB group vs. 41.3% of the placebo group at a similar rate [48].
CBD has been evaluated for its anxiolytic properties in an RCT, double-blind placebo-controlled, in which 80 patients (aged 18-65 years) with a treatment-refractory social anxiety disorder or panic disorder with agoraphobia were enrolled.Patients underwent eight therapist-assisted exposure in vivo sessions (weekly, outpatient).Prior to each session, patients were administered 300 mg oral CBD (n = 39) or placebo (n = 41).Although no differences were found in treatment outcome over time between CBD and placebo as measured by the Fear Questionnaire assessed at baseline, mid-, and post-treatment, and at 3-and 6-month follow-up, only a few AEs were reported, evenly distributed between the CBD (n = 4) and the placebo groups (n = 6).Most reported AEs were dizziness, drowsiness, recurrent tiredness, the recurrent feeling of a strong blood flow, and recurrent headache [58].In an open-label study enrolling 31 young subjects aged 12-25 years with treatment-refractory anxiety disorder, anxiety severity scores significantly decreased after receiving CBD for 12 weeks at 400-800 mg.CBD demonstrated an acceptable safety profile with no serious AEs reported.Most patients (80.6%) experienced mild or moderate AEs.The most common AEs were fatigue, low mood, hot flashes, and cold chills [63].A more recent open-label trial used CBD for the reduction of emotional an effective agent for the reduction of emotional exhaustion and burnout symptoms in frontline healthcare professionals working with patients with COVID-19.A total of 120 participants were randomized to receive either CBD, 300 mg, plus standard care (treatment arm; n = 61) or standard care alone (control arm; n = 59) for 28 days.Five participants who received CBD plus standard care experienced serious AEs, including increased liver transaminase levels and skin rash, with full recovery after discontinuation [66].Most AEs were mild or moderate in severity (> 10%).Somnolence (n = 34 (28.8%)),fatigue (n = 27 (22.9%)),increased appetite (n = 19 (16.1%)), diarrhea (n = 13 (11.0%)),weight gain (n = 12 (10.2%)),lethargy (n = 12 (10.2%))were frequently reported.
In a retrospective case series, data from 11 patients diagnosed with post-traumatic stress disorder were evaluated to investigate CBD efficacy and safety profile.CBD was given in a flexible dosing regimen in two different formulations, capsular and liquid spray (n = 4 only capsular; n = 1 only liquid spray; n = 6 both capsular and liquid spray).The mean total starting dose of CBD (liquid or capsular or both) was 33.18 mg (standard deviation = 23.34).The mean total dose of CBD at the conclusion of the study period was 48.64 mg (range 2-100).Patients were taking concomitant medications of different classes; the most relevant were AEDs, antidepressants, antipsychotics, anxiolytic/sedatives, and betablockers.CBD was well tolerated with no serious AEs reported and few mild/moderate AEs [65].
Likewise, CBD showed an acceptable safety profile in an infantile population with severe behavioral problems and intellectual disabilities in an RCT, double-blind, placebocontrolled trial conducted by Efron et al. [59].Eight children aged 8-16 years receiving oral CBD at a maximum dose of 500 mg/day reported only a few mild/moderate AEs.No reduction adjustments due to AEs were necessary, and no withdrawals or serious AEs were recorded [59].No significant serious AEs were also reported by studies evaluating oral CBD at a range dose of 200-800 mg for a duration of 3 to 28 days in a cohort of subjects with different types of addiction, such as cannabis use disorder [57], opioid use disorder [60,64], cocaine use disorder [61], and tobacco use disorder comorbid to psychotic syndromes [19].Across the studies, most AEs reported were mild or moderate in severity.Nausea, diarrhea, fatigue, tiredness, and headache were commonly reported.

Drug-Drug Interaction and Pharmacokinetic Studies
CBD is metabolized in the liver and the intestine by cytochrome P450 (CYP) CYP2C19 and CYP3A4, and 5'-diphosphoglucuronosyltransferase (UGT) UGT1A7, UGT1A9, and UGT2B7 isoforms, mainly producing hydroxylated and carboxylated metabolites.CBD inhibits barbiturate metabolism, increasing barbiturate-induced sleep duration in mice, and phenazone hepatic metabolism [67] due to the inhibition of CYP3A and CYP2C microsomal enzymes [68].It has been suggested that CBD also induces hepatic CYP3A, CYP2B, and CYP2C [69].Considering the complexity of CBD pharmacology, available clinical trials address the occurrence of AEs and DDIs in a cohort of healthy subjects [70][71][72][73][74][75][76][77].Liver safety has been investigated in an open-label DDI trial enrolling 16 healthy subjects aged 18-60 years [76].Therapeutic oral CBD at a dose of 1500 mg was administered daily for about 3.5 weeks, and the effects on CYP1A2 activity were monitored.Of 16, seven subjects (44%) experienced peak serum alanine aminotransferase values greater than the upper limit of normal.Five subjects (31%) reached the international consensus criteria for druginduced liver injury.There was no correlation between transaminase elevations and baseline characteristics, CYP2C19 genotype, or CBD plasma concentrations.All alanine aminotransferase elevations above the upper limit of normal began within 2-4 weeks of initial exposure to CBD.DDIs and liver impairment may occur mostly when CBD is coadministered with AEDs, such as clobazam, valproate, and stiripentol, as they share similar metabolic pathways.An open-label DDI study investigated the reciprocal impact on the steady-state pharmacokinetics of CBD, clobazam (and Ndesmethylclobazam), stiripentol, and valproate; and CBD safety and tolerability when co-administered with each AED.Concomitant cannabidiol had little effect on clobazam exposure, N-desmethyl clobazam exposure increased, and stiripentol exposure increased slightly, while no clinically relevant effect on valproate exposure was observed.Concomitant clobazam with CBD increased 7-OH-CBD exposure, without notable 7-COOH-CBD or CBD increases.Stiripentol decreased 7-OH-CBD exposure by 29% and 7-COOH-CBD exposure by 13%.There was no effect of valproate on CBD or its metabolites.CBD was moderately well tolerated, with similar incidences of AEs with concomitant clobazam, stiripentol, or valproate.There were no deaths, serious AEs, pregnancies, or other clinically significant safety findings [77].
A pharmacokinetic DDI trial investigated the effects of repeated dosing of oral CBD on caffeine clearance via modulation of CYP1A2 activity in healthy adults.Sixteen healthy subjects received a single 200 mg caffeine dose and placebo on day 1.CBD was then titrated to reach the dosage of 750 mg twice daily through 28 days of treatment.CBD affected the exposure of caffeine, a CYP1A2 substrate, leading to an elevation of 15% for C max , 88% for AUC 0-t , and 95% for AUC 0-∞ , and showed a good safety profile with no unexpected AEs.Diarrhea was the most common, and six subjects discontinued because of elevated liver transaminases [75].
Two studies with a crossover or RCT double-blind design focused on the pharmacokinetic effects of different oral [70] or sublingual formulations of CBD [72].Fourteen healthy subjects, aged ≥ 18 years (mean 26 [8]), were randomized to take each visit (5 in total) a different CBD formulation: (725 Water soluble.Contains sorbitol; 088 Not water soluble.Contains medium chain triglyceride coconut oil; 126 Water soluble.Contains gum Arabic and maltodextrin; 213 water-soluble.Contains gum Arabic and sorbitol; 625 Not water soluble.Pure CBD as a crystalline powder (>99% purity)) [70].Comparing the pharmacokinetics of (CBD) and CBD metabolites (6-OH-CBD, 7-OH-CBD, and CBD-COOH) following ingestion of five different CBD formulations, considerable variability was found among the different formulations.When delivered in an aqueous beverage, water-soluble CBD formulations typically evoked the greatest exposure.CBD was not influencing the thermic effect of food; however, it did lower insulin and triglyceride concentrations during the first 30-min following food ingestion.Moreover, a single 30 mg dose of CBD did not influence most of the circulating markers of liver and kidney function.Consistently, no serious AEs were reported [70].The pharmacokinetic properties and safety profile of a novel CBD wafer formulation compared with an oil formulation were evaluated by Hosseini et al. [72].First, they conducted an open-label, single-dose, crossover study in which 12 healthy volunteers were randomized to receive a sequence of four different single doses of CBD as a sublingual wafer (25 or 50 mg CBD), oil solution (50 mg CBD), or nabiximols oromucosal spray (20 mg CBD, 21.6 mg THC).Then, they carried out a double-blind study where a sublingual wafer (50 mg CBD) was administered twice a day for five days.The extract was generally well tolerated by participants when administered in either wafer or oil form, with some adverse events, including mild or moderate somnolence, sedation, and altered mood.The median maximum concentrations of CBD after administration of the oil solution and wafer were 9.4 and 11.9 ng/mL, respectively.Maximum concentrations of CBD occurred 4 hours after administration, with an estimated terminal elimination half-life of 6 hours [72].
Pharmacokinetic, safety, and tolerability of oral CBD have been evaluated in healthy volunteers after the consumption of different meal compositions.A new lipid oral CBD formulation was administered at doses of 5, 10, and 20 mg/kg in 24 healthy volunteers randomized for each CBD dose, and placebo [73].This RCT double-blind, placebocontrolled study demonstrated the new CBD formulation is well tolerated even at an escalating dose with no serious AEs reported.AEs of mild or moderate severity were reported by 42% of subjects; the most common being headache (17%) (CBD group 3/18 versus placebo group 1/6) and diarrhea (8%) (CBD group 3/18 versus placebo group 0/6).The CBD plasma exposure increased in a dose-proportional manner and declined to levels approaching the lower level of quantification by day 8.The terminal elimination half-life was approximately 70 h, suggesting that 2-3 weeks are needed to fully eliminate CBD [73].Similar findings were found in an open-label crossover study using the highly purified formulation of CBD (Epidiolex).A single CBD dose of 750 mg was administered in thirty healthy subjects following a highfat/calorie meal (n = 15), a low-fat/calorie meal (n = 14), whole milk (n = 15), or alcohol (n = 14), compared to the fasted state (n = 29).CBD plasma exposure increased under all test treatment conditions in this trial when compared to the fasted state.No serious AEs were recorded.Mild or moderate AEs were reported in 90% (27/30) of subjects.Only two (13%) subjects experienced moderate AEs after administration with alcohol.The most common AEs were headache in 13 (43%) subjects overall, and somnolence and diarrhea in five (17%) subjects overall.Many more subjects were affected by headaches while taking CBD with alcohol (eight (53%) subjects) versus the other treatment groups (7%-21% subjects affected).An increased incidence of dizziness was found in subjects while taking CBD with alcohol [71].
Finally, an RCT, double-blind, placebo-controlled trial assessed whether withdrawal symptoms occurred after the abrupt cessation of CBD after prolonged administration in 30 healthy volunteers.No evidence of withdrawal syndrome was found with abrupt discontinuation of short-term treatment with 750 mg CBD twice daily (b.i.d.) for 4 weeks (Part 1) followed by 2 weeks of 750 mg b.i.d.CBD (Part 2, Arm1) or matched placebo (Part 2, Arm2).No serious AEs were reported.Mild or moderate AEs reported were consistent with those reported by other clinical trials [74].
Interestingly, an open-label trial was conducted to assess the pharmacokinetics and safety of CBD in subjects with mild to severe hepatic impairment.The population group consisted of 30 subjects, aged 18-75 years.Alongside CBD, they consumed common concomitant medications, such as beta-blocking agents, diuretics, and drugs for acid-related disorders.The trial consisted of subjects being administered oral CBD 200 mg daily for 3 days.This resulted in common AEs, varying between groups with mild and moderate hepatic impairment.The subject group with mild hepatic impairment had one case of diarrhea and dizziness, while the subject group with moderate hepatic impairment had one case of diarrhea and a low platelet count.However, there was no case of AEs in the subject group of severe hepatic impairment.Neither was an increase in AEs severity or frequency noted with the elevating degree of hepatic impairment [78].
Patients who receive CBD may have co-existing renal disturbances.Therefore, it is important to understand whether dose adjustments are necessary to mitigate the risk of exposure-related toxicity.To gain a deeper insight into the pharmacokinetics, safety, and tolerability of CBD in subjects with mild to severe renal impairment, a phase I, open-label, parallel-group, a single-dose trial was conducted [79].The subject group was selected to be 32 subjects, ages 18-75 with either mild (n = 8), moderate (n = 8), or severe (n = 8) renal impairment as well as some with normal renal function (n = 8).All groups equally of 8 subjects received 200 mg of oral CBD as a single dose and were followed up after 15 days.In addition to the CBD dose, they consumed concomitant medications including blood pressure-regulating agents (βblockers, xanthine oxidase inhibitors, calcium channel blockers), thyroid hormones, and diuretics.None were considered to impact the safety or interpretation of the trial data.In the trial outcome, the subjects had defined all AEs as mild.Only five mild AEs were reported by two subjects, one in the mild renal impairment group and one in the normal renal function group.There was no increase in AE frequency or severity with an increasing degree of renal impairment.

Pain Management
CBD has been also evaluated for its analgesic and antiinflammatory properties.Alaia et al. demonstrated that buccally absorbed CBD has good efficacy for pain control right after arthroscopic rotator cuff repair in an RCT, doubleblind, placebo-controlled trial.Ninety-nine opioid-naive subjects, aged 18-75, undergoing arthroscopic rotator cuff repair were randomized in placebo (n = 47) and CBD groups (n = 54).For 14 days, subjects received 25 or 50 mg/kg/day CBD alongside opioids as their concomitant medication.One AE of increased transaminase level was experienced in both groups, 4/45 in the CBD group, and 4/47 in the placebo group [80].Furthermore, a within-subject, randomized, placebo-controlled, double-blind study was carried out to present the analgesic effects, abuse liability, safety, and tolerability of acute CBD [22].A group of 17 healthy noncannabis-using subjects, aged 21-50 years were receiving a single dose of 200, 400, or 800 mg oral CBD in a randomized order.With no other concomitant medications reported, all AEs were mild to moderate.There were no differences in AEs between the placebo and CBD groups.Common AEs reported in both groups were lethargy and upset stomach.Subtle mood changes, frequent urination, and wooziness were also reported.
Additionally, an RCT, double-blinded, placebo-controlled trial was performed to examine CBD as an add-on analgesic treatment option for subjects with hand osteoarthritis and psoriatic arthritis [81].A group of 129 subjects was given synthetic CBD 20 to 30 mg or a placebo daily for 12 weeks.The CBD group comprised 70 subjects with a mean age of 62 years, while the placebo group entailed 66 subjects with a mean age of 61.50 years.The subject group received 10-20 mg/kg/day of either a CBD or placebo tablet orally.Alongside, they also consumed concomitant medication including paracetamol, non-steroidal anti-inflammatory drugs, antileptics, codeine, tramadol, and opioids.Four serious cases of AEs were reported including acute shoulder fracture, malignant hypertension, ductal carcinoma, and lipothymia that were not deemed to be CBD-related.Finally, the therapeutic potential of a topical CBD formulation with shea butter (6.2 mg/mL) has been evaluated for the treatment of pain associated with thumb basal joint arthritis [82].Eighteen subjects were randomized in a crossover, double-blind, placebocontrolled study design and received topical CBD (6.2 mg/mL CBD with shea butter) or shea butter alone for two weeks.No adverse events were reported, and topical CBD resulted effective in pain control [82].
Since CBD may exert anti-inflammatory and antiviral properties, CBD has been investigated as a potential treatment option for coronavirus disease 2019 (COVID-19).An RCT, double-blind, placebo-controlled trial was carried out for two weeks.The trial involved 109 subjects with mild to moderate symptoms of COVID-19 related [62], subdivided into 49 subjects in the CBD group, who were administered oral CBD at a dosage of 300 mg daily, and 42 subjects in the placebo group.Subjects were taking concomitant medication such as paracetamol, acetaminophen, and dipyrone.Most AEs were mild to moderate.Somnolence, fatigue, decreased appetite, lethargy, weight loss, and diarrhea were commonly reported.These AEs were transient with no statistically significant differences between the placebo and CBD groups.

DISCUSSION
The therapeutical potential of drugs modulating the activity of endocannabinoid receptors has drawn substantial interest [23].Since the approval of the highly purified formulation of CBD, Epidiolex, in 2018, a growing number of studies were conducted to investigate the efficacy and the safety of CBD in several medical conditions.We previously reviewed the literature regarding CBD pharmacokinetic properties, tolerability, and safety profile in both preclinical and clinical studies [28].The main aim of the present work was to offer an updated review of the available evidence regarding the use of CBD in the clinical context.Compared to our previously published article [28], new data have become available on the long-term efficacy and tolerability of CBD for the treatment of TREs suggesting that CBD has a good safety profile in both children and adults' populations.Interestingly, in the last few years, a larger number of clinical trials have been conducted for multiple clinical indications, outside of LGS and DS, including psychosis, anxiety disorders, addictions, and pain management.Moreover, many ongoing clinical trials are evaluating the efficacy and the safety profiles of CBD in conditions such as Alzheimer's disease, several forms of chronic pain, fibromyalgia, developmental and behavioral disorders, and liver and heart disorders in an effort to better understand CBD pharmacodynamics and pharmacokinetics, and its therapeutic potential.Overall, this updated systematic review provides new evidence confirming a well-tolerated safety profile and suggesting that CBD may represent a new therapeutic approach as a stand-alone or an add-on drug for several unmet clinical needs.Across all included trials, most patients reported the occurrence of at least one AEs.Most AEs were mild or moderate in severity.The few serious AEs reported were frequent causes of treatment discontinuation or hospitalization with a full recovery after CBD interruption.CBD potently inhibits hepatic metabolism by acting on CYP3A4 and CYP2C19.These enzymes also metabolize several AEDs that are often taken concomitantly with CBD, such as clobazam, valproate, and stiripentol, in a cohort of patients enrolled in clinical trials focusing on TRE.Oral CBD, when prescribed at higher doses (10-20 mg/kg/day), was associated with an increased rate of serious AEs, among which elevation of liver transaminase, somnolence, sedation, and respiratory infections were most commonly reported.The majority of cases with increased liver transaminase were in children or adults with epilepsy who were also being treated with sodium valproate.The exact mechanism of this interaction is not fully understood.CBD and valproate co-administration does not significantly alter their plasma levels or of their metabolites [77].However, CBD metabolite 7-COOH-CBD and valproate and its metabolite 4-ene-valproic acid may affect hepatic mitochondrial function.Similarly, concomitant CBD did not significantly affect clobazam plasma exposure in healthy subjects [77].Given CBD complex and multimodal pharmacology with a potential increase of side effects when taken with clobazam, two metanalyses evaluated the efficacy and tolerability of CBD with or without concomitant clobazam, providing conflicting results.Indeed, a metanalysis of four phase III, RCTs, double-blind, placebo-controlled trials (GWPCARE1, 2, 3, 4) [83] found that the safety profile of oral highly purified CBD (10 or 20 mg/kg/day for a 14-week treatment) was affected by the concomitant administration of clobazam.Reported AEs and serious AEs were more frequent in patients taking both CBD and clobazam than in those taking CBD without clobazam and placebo.Somnolence was frequently reported by patients with concomitant clobazam and increased at higher CBD doses [83].Conversely, Gunning et al. did not find an increased incidence of AEs in LGS and DS patients taking concomitant clobazam [84].Somnolence was the most frequent AE reported in 22% of patients with LGS and 27% of patients with DS, of whom 67% and 84%, respectively, were taking concomitant clobazam, and was not considered dose related.Other common AEs, such as diarrhea, decreased appetite, and vomiting were not more common in patients on clobazam.Serious AEs related to CBD and their possible relation to DDIs were analyzed by a systematic review of RCTs involving the administration of oral CBD for at least one week in healthy subjects and in clinical populations, including patients with epilepsy, PD, social anxiety disorder, and psychosis [85].A higher incidence of serious AEs was observed in epilepsy RCTs.Somnolence, sedation, the elevation of liver transam-inase level, skin rash, and respiratory disturbances were the most commonly reported serious AEs.These effects seem to be related to DDIs, especially with valproate in the case of increased liver transaminase level and with clobazam in the case of sedation and somnolence.No significant serious AEs were found in studies conducted on healthy subjects or patients with PD, Huntington's disease, type 2 diabetes, Crohn's disorder, and social anxiety disorders, even considering the concomitant medications [85].These findings were further confirmed by a systematic review and metanalysis of CBD AEs across all medical indications suggesting that CBD is well tolerated and induces few serious AEs, whose rate increases when combined with other drugs in childhood epilepsy populations [86].Finally, the pharmacokinetic interaction between CBD and mTOR inhibitors has been evaluated only in one retrospective chart review.The plasma level of everolimus and sirolimus increased significantly when taken with CBD [44].However, the overall severity and prevalence of AEs were similar to those identified in RCTs.Further research and clinical studies should evaluate the pharmacokinetic interactions of CBD with several classes of medications, as the clinical indication of CBD is expanding.New therapeutic drug monitoring methods should be developed to help clinicians in the clinical management of patients.Indeed, more accurate detection of CBD and its metabolites may help the dose adjustments and reduce the rate of AEs especially when CBD is co-administered with other drugs [87,88].
Critical factors to evaluate CBD safety and tolerability are the dose, the route of administration, and the frequency of use.In the included trials, the CBD dose ranged from 2.5 to 50 mg/kg/day as a single or multiple administration.Importantly, compared to our previous review on CBD toxicity and AEs [28], more data are available on long-term CBD use, particularly in clinical trials in epileptic children and adults for up to 36 months.Moreover, clinical studies have focused on CBD antiseizure properties in other forms of refractory childhood epilepsy, expanding its therapeutic application.
Only three-open label studies aimed at evaluating CBD effects on cognition.Oral CBD, at the common dosages used in clinical trials, seems to not affect cognitive functions.These preliminary findings may pave the way for an expansion of CBD clinical indications.Interestingly, CBD has recently been under investigation as a potential add-on treatment for neurological disorders, such as PD and AD.In a study involving PD patients with rapid eye movement behavior disorder, although CBD was ineffective to reach the primary outcome, it showed a good safety profile with no significant AEs compared to a placebo.Similar findings were reported by studies investigating the therapeutic role of CBD in the adult cohort population with anxiety disorders, addictions, and chronic pain conditions.Most of the included studies have used the highly purified formulation of CBD Epidiolex that is administered orally.Few well-designed clinical trials are still available on different formulations and routes of administration.In studies using a transdermal gel formulation, CBD dose ranged from 50 to 500 mg/day.Most patients experienced at least one AE of mild or moderate severity.No significant serious AEs were reported.The most common AEs were gastrointestinal and respiratory disturbances and somnolence.An increased rate of skin rash compared to studies using oral CBD was reported.Only single, open-label studies evaluated different formulations of CBD, including smoking CBD cigarettes, liquid spray, and sublingual wafers, reporting an acceptable safety profile.However, further well-designed and controlled trials are needed to support the therapeutic use of these CBD formulations for medical purposes.
A concerning issue in the CBD field is the availability of over-the-counter (OTC) CBD products, which have not been evaluated in clinical trials.OTC preparations may contain CBD at different doses along with other cannabinoids and/or other contaminants [89][90][91].This issue requires more research, as the market of OTC CBD products is poorly regulated.

CONCLUSION
CBD has shown a well-tolerated safety profile with few AEs.However, its ability to inhibit the hepatic metabolism of other drugs, such as clobazam, valproate, and stiripentol, can potentially cause AEs.Indeed, an increased rate of AEs, serious AEs, and discontinuation of treatment has been demonstrated in several clinical trials focusing on TRE in children.Data from clinical studies involving adult subjects with disease conditions are still limited.Therefore, additional data on CBD tolerability and safety from clinical studies outside childhood epilepsy syndromes, studies involving subjects with different medical conditions, and studies of over-the-counter CBD products are needed.

STANDARDS OF REPORTING
PRISMA guidelines and methodology were followed.